The present invention relates to a friction stir welding method that is suitably applied to panel welding used, for example, in aluminum alloys railway cars and buildings.
A two-face structure (panel) for railway cars using hollow is disclosed in Japanese Patent Laid-Open No. 246863/1990, and another using laminated panels, such as honeycomb panels, is disclosed in Japanese Patent Laid-Open No. 106661/1994.
The process of friction stir welding is performed by rotating a round rod inserted in a joint region to heat and plasticize the joint region thus forming a weld. This type of welding is generally applied to a butt joint and a lap joint, and is described in WO 93/10935 (which is the same as EP 0615480B1 and the Japanese Laid-Open Publication No. Hei 7-505090) and in the Publication Welding and Metal Fabrication, January 1995, pp. 13-16.
In the friction stir welding, the reaction of the plasticized metal being extruded from immediately beneath the rotating tool (round rod) to the surface during the welding results in a downward force acting on the joint region. Thus, when this welding method is applied to a two-face structure (panel), this downward force causes the joint material at the joint region to flow downward, deforming the joint. This makes it impossible to produce a satisfactory weld.
Two-face structures (panels) include hollow members made of extruded aluminum alloy and honeycomb panels. Joining such panels has been accomplished by MIG welding and TIG welding. When friction stir welding is applied to such a joint, the joint is bent down or the material in the joint region is forced to flow down due to a downward force produced during the friction stir welding.
The inventor has found the above phenomena in a variety of experiments.
It is a first object of this invention to provide a satisfactory welded joint by minimizing deformation of the joint region when two faces are friction stir welded.
It is a second object of this invention to provide a satisfactory welded joint when one face is friction stir welded.
It is a third object of this invention to enable two faces to be welded together in a short time with little deformation.
The first object can be achieved by placing at the joint region a connecting member that joins two plates forming two faces.
The second object is realized by providing the members at the joint region with a raised portion that protrudes toward the friction stir welding tool side.
The third object is realized by disposing rotary tools for friction stir welding on both sides of the objects to be welded, placing the rotation center of one of the tools on an extensions of the rotation center of the other tool, and performing friction stir welding simultaneously.